||The boogie woogie illusion.
A grid of vertical and horizontal lines, each composed of light and dark squares, is moved rigidly at 45 deg to the vertical on a gray surround. When the luminance of the background is set midway between the luminances of the light and dark squares, the squares appear to race along the lines even though they are actually "painted" on the lines. The effect arises from the unequal apparent speeds of the lines and their textures. The light and dark squares along the lines define a 1st-order pattern whose apparent speed, parallel or along the line, is close to veridical. The lines themselves have no overall luminance difference from the background so that they are defined by a 2nd-order difference. Apparent speed is reduced for 2nd-order motion so that the motion perpendicular to the line is perceived as slower than the motion along the line even though they are physically equal. The imbalance creates the impression that the small squares are moving along the lines rather than moving rigidly with them. (PsycINFO Database Record (c) 2007 APA, all rights reserved)